This disclosure generally relates to test equipment for photovoltaic cells and, more particularly, relates to solar simulator systems used in testing of solar (i.e., photovoltaic) cells.
Solar cells convert the sun's energy into useful electrical energy by way of the photovoltaic effect. Modern multi-junction solar cells operate at efficiencies significantly higher than traditional, silicon solar cells, with the added advantage of being lightweight. Therefore, solar cells provide a reliable, lightweight and sustainable source of electrical energy suitable for a variety of terrestrial and space applications.
During the design and manufacture of solar cells, there is often a need to test solar cells for power generation and overall operating efficiency. One option for testing solar cells is exposing the solar cells to natural sunlight, as if the solar cells were in deployment. However, for a variety of reasons, it is often not practical (or even feasible) to expose test solar cells to natural sunlight.
Thus, solar simulators have been developed as an alternative to testing solar cells with natural sunlight. Advantageously, solar simulators facilitate the indoor testing of solar cells under controlled laboratory conditions. Unlike solar cells designed for outer space applications, terrestrial solar cells can be exposed to sunlight that is “filtered” through different atmospheric and/or environmental conditions. Moreover, the altitude at which the solar cells will be deployed can influence the spectral (wavelength) characteristics of sunlight. Consequently, a solar simulator should be configured to provide accurate spectral adjustability to simulate different types of sunlight conditions.
Large-area solar simulators depend on spatial uniformity to make accurate and repeatable measurements of solar cells. However, solar simulators do not always provide a smooth illumination area for testing of large solar cells. This spatial non-uniformity causes errors to be created during the measurements of solar cells. This is particularly important when reference cells are used to compensate for temporal instabilities in the illuminating beam. (A reference cell is a calibrated photovoltaic or solar cell.)
One solar simulation system uses reference cells (located in a different part of the illuminating beam) to simultaneously measure the illumination in one area of the illuminating beam and assumes that illumination value is representative for the entire illuminating beam. Reference measurements are actively taken at one location in the illuminating beam and the solar cell under test (also referred to herein as “device under test (DUT)) is placed at a different part of the illuminating beam. Spectral balancing and absolute irradiance is “measured” by the reference cell and it is assumed that the solar cell under test has the same spectral balance and same absolute irradiance. When spectral filtering or focusing is done, the spatial distribution across the illumination plane changes and is not measured with this system. Misalignment or poor design can produce spatial non-uniformities of 5% and larger across the illuminated area at the illumination plane.
It would be desirable to provide means and methods for compensating for spatial non-uniformities in solar simulators.